1. Field of the Invention
This invention relates to a process for converting mixtures of sodium carbonate and sodium bicarbonate, such as trona, into sodium carbonate. More particularly, this invention relates to two- and threecompartment electrodialytic water splitting processes for converting materials comprising sodium carbonate and sodium bicarbonate into a liquid comprising aqueous sodium and hydrogen ions, and a liquid comprising sodium carbonate substantially free of sodium bicarbonate from which high purity, high strength sodium carbonate can be recovered in high yields.
2. Description of the Prior Art
Most soda ash (sodium carbonate) produced in the United States is obtained from naturally occurring subterranean trona ore deposits located in southwestern Wyoming. Trona ore consists mainly of sodium sesquicarbonate (Na.sub.2 CO.sub.3. NaHCO.sub.3.2H.sub.2 O, a hydrated sodium carbonate sodium bicarbonate double salt) and normally contains 4-13% insoluble impurities. A typical analysis of crude trona ore is:
______________________________________ PERCENT BY WEIGHT ______________________________________ Na.sub.2 CO.sub.3 41.8 NaHCO.sub.3 33.1 H.sub.2 O 14.1 NaCl 0.04 Na.sub.2 SO.sub.4 0.01 Iron 0.08 Water insolubles 10.87 ______________________________________
The composition of the crude trona ore corresponds quite closely to that of pure sodium sesquicarbonate except for the impurities present. Shale stringers or beds, normally present throughout a trona bed, will alter the amount of impurities in crude trona ore.
At the present time, these trona deposits are normally mechanically mined. For example, dry-mined trona ore is converted to soda ash by either a sesquicarbonate process or a monohydrate process (the features of both processes are summarized in U.S. Pat. No. 3,528,766 to Coglaiti et al). In the sesquicarbonate process, the trona ore is dissolved in hot aqueous alkali solution and, after separation of the resulting solution from the insolubles, sodium sesquicarbonate is crystallized from solution by cooling. The sesquicarbonate crystals are then separated from the mother liquor and, finally, calcined to recover soda ash (anhydrous sodium carbonate). In the monohydrate process, the trona ore is first calcined to convert its bicarbonate content to sodium carbonate and is then dissolved in water. After the resulting solution is separated from the insolubles, sodium carbonate monohydrate is precipitated by evaporative crystallization. The monohydrate crystals are then separated from the mother liquor and dried to recover soda ash.
While relatively effective, these processes suffer from several inherent disadvantages which seriously affect the efficiency and commercial viability of the processes. For example, because of the need for calcination steps, the processes are energy intensive (requiring the use of large amounts of coal, fuel oil, natural gas or mixtures thereof). This is especially disadvantageous because of the high cost and scarcity of hydrocarbon fuels. Furthermore, these procedures are not readily adapted for removal of soda ash from uncalcined trona values in dilute aqueous streams, as for example solutions obtained via direct solution mining, mine waters, or from calcined trona values in aqueous process streams such as pond liquors or crystallizer mother liquor purges.
Recently, solution mining techniques have been utilized as an alternative to dry-mining to recover soda ash from subterranean trona ore deposits Solution mining of subterranean trona deposits by using hot water or various alkaline solutions is well known. For example, U.S. Pat. No. 2,388,009 (Pike) discloses the use of a hot water or hot carbonate solution as the mining fluid See also U.S. Pat. Nos 2,625,384 (Pike et al.); 2,847,202 (Pullen); 2,979,315 (Bays); 3,018,095 (Redlinger); 3,050,290 (Caldwell et al.); 3,086,760 (Bays); 3,405,974 (Handley et al.) and 4,288,419 (Copenhafer et al.). Solution mining of subterranean trona deposits by using aqueous sodium hydroxide is disclosed by U.S. Pat. No. 3,184,287 (Gancy), U.S. Pat. No. 3,952,073 (Kube) and U.S. Pat. No. 4,344,650 (Pinsky et al.). U.S. Pat. No. 4,283,372 (Frint et al.) discloses the use of an aqueous ammonia solution as a mining fluid for trona These prior art solution mining processes, however, require substantial amounts of energy to manufacture sodium hydroxide, manufacture and recycle ammonia, supply high temperature mining solutions and, in some processes, calcine one or more intermediates.
In addition to trona, nahcolite (predominantly NaHCO.sub.3) and wegscheiderite (predominately Na.sub.2 CO.sub.3.3NaHCO.sub.3) are also sodium bicarbonate-containing ores from which it is possible to recover soda ash, after conversion of the bicarbonate to carbonate. Known deposits of nahcolite and wegscheiderite are located primarily in Utah and Colorado. No commercial operations are presently known to be recovering soda ash from these NaHCO.sub.3 -bearing minerals. Various U.S. patents disclose solution mining of nahcolite: for example, U.S. Pat. No. 3,779,602 (Beard et al.) and U.S. Pat. Nos. 3,792,902 (Towell et al.). In addition, U.S. Pat. No. 3,952,073 (Cube) and U.S. Pat. No. 4,283,372 (Frint et al.) disclose basic solution mining of nahcolite and wegscheiderite.
Electrochemical methods have been proposed for converting dilute aqueous solutions into usable products. Electrodialytic water-splitting is a process which has been successfully employed to recover valuable products from such dilute streams. For example, U.S. Pat. No. 4,082,835 (Chlanda et al.) discloses an electrodialytic process which utilizes two- or three-compartment water-splitters to remove SO.sub.2 from dilute gas streams by means of (a) alkaline solution scrubbing, (b) regeneration of the scrubbing solution and, (c) liberation of concentrated SO.sub.2. Another electrodialytic process is disclosed in U.S. Pat. No. 4,389,293 (Mani et al.) for the recovery of anhydrous hydrogen fluoride from dilute aqueous solutions of fluosilicic acid and hydroflouric acid.
An electrodialytic process for converting aqueous streams of trona into valuable products has been described in U.S. Pat. No. 4,238,305 (Gancy and Jenczewski). In this process, dilute aqueous trona is fed to the acid compartment of an electrodialytic cell for conversion into sodium hydroxide and carbon dioxide. The reaction products are either recovered and used separately, or combined in another reaction zone to provide soda ash. While relatively effective, the process requires that H.sub.2 CO.sub.3 /CO.sub.2 be generated and liberated within the acid compartment which, consequently, increases the electric power necessary for the electrodialytic process.